Galaxy Zoo and undergraduate research: spiral arms, colors, and brightnesses
The guest post below is by Zach Pace, an undergraduate physics student at the University of Buffalo. Zach worked at the University of Minnesota during the summer of 2013 through the NSF’s Research Experience for Undergraduates (REU) program. Zach is continuing to work with Galaxy Zoo data as part of his senior thesis.
Hi, everyone–
My name is Zach Pace. I’m an undergraduate physics student from the University at Buffalo, and I’ve been working on the Galaxy Zoo 2 project at the University of Minnesota since late May with Kyle Willett and Lucy Fortson. My investigation has been twofold: I have been diagramming specific morphological categories in color-magnitude space, and also fitting those data to mathematical functions.
As many readers probably know, a galaxy’s magnitude (overall brightness in the red band, on a log scale) and a galaxy’s color (the difference between the blue magnitude and a red band) are two important quantities for determining what a galaxy might look like (and how it might evolve). Brighter galaxies have more mass (more stars produce more light, of course), and bluer galaxies have a more recent star formation history (this is because young, bright stars tend to be large, bright, and blue). In terms of the whole population, we know, for instance, that elliptical galaxies tend to concentrate in a red sequence, and have typical colors between 2.25 and 2.75. Conversely, the vast majority of spiral galaxies concentrate in a blue cloud between colors 1.25 and 2.0. These two populations are clearly separated in color-magnitude space (this can be seen in the accompanying 2-D histogram, made from Zoo 2 data).
Color-magnitude diagram (CMD) for objects in Galaxy Zoo 2. The lines show fits to the two main populations of elliptical (red) and spiral (blue) galaxies, following the method of Baldry et al. (2004). The green line shows an approximate separation between them.
One of the main goals of Zoo 2 is to gauge the extent to which morphology informs physical characteristics like color and magnitude, so my objective for the summer was to come up with good representations of color and magnitude for all of the smaller sub-populations in Zoo 2.
Several of my results were interesting and surprising. For instance, it has been suggested that spiral galaxies with more arms and spiral galaxies with tighter arm winding (which is to say, a shallower pitch angle) tend to be brighter and bluer. This can be intuitively understood as follows: tighter winding of spiral arms and the presence of more spiral arms indicate, on average, denser gas clouds in those arms, which is tied to increased star formation and bluer color. However, I wasn’t able to measure this in the Zoo 2 data (all the differences were on the order of the histograms’ bin size, about 0.1 magnitude, or about a 10% difference in brightness). This suggests that spiral galaxies, no matter arm multiplicity or winding, are drawn from the same base population.
Color-magnitude diagram (CMD) for spirals in GZ2, split by the number of spiral arms identified in each galaxy. The distribution of colors and magnitudes for galaxies are statistically similar, no matter what the number of spiral arms.
I also came across something unexpected when looking at bulge sizes in face-on disk galaxies. The distribution of galaxies classified by users as bulgeless is starkly different from the distribution of obvious bulge and bulge-dominated galaxies. Furthermore, the population with a bulge that is just noticeable seems to form an intermediate population between the bulgeless and bulge. This observation is also borne out in edge-on disk galaxies: the population of bulgeless edge-on galaxies has a similar shape to the population of face-on galaxies, albeit with stronger reddening on the bright end.
Color-magnitude diagram (CMD) for disk galaxies in Galaxy Zoo 2, split by the relative size of the central bulge. Galaxies that appear to have no central bulge (top) have very different colors and luminosity than those with dominant bulges (bottom).
To fit the distributions, I used a method pioneered about 10 years ago by Ivan Baldry, which fits one parameter after another in our profile functions to find a distribution that converges onto the best fit. It works okay (but not great) for the whole sample, and it fails pretty badly when working with the smaller sub-populations. This is because I have to fit many parameters at once, and do that a bunch of times in a row for the fit to converge, so there are a lot of points of failure. I’m working now at Buffalo towards finding a different and better fitting routine, which will allow us to represent more distributions mathematically.
If you have any questions, feel free to comment below.
Galaxy Zoo 
Welcome Zach, and thanks for a great blog post!
Would you mind spelling out in some more detail some of the things in your blog post please?
For example, “the difference between the blue magnitude and a red band” – I guess you mean the SDSS u’ and r’ band magnitudes, corrected for galactic (MW) extinction, right? If not, what?
Also, “ it has been suggested that spiral galaxies with more arms and spiral galaxies with tighter arm winding (which is to say, a shallower pitch angle) tend to be brighter and bluer” – with the caveat “controlled for inclination” (or similar), right? I mean, comparing a highly inclined spiral to a face-on, without “correcting” for inclination, could yield spurious results, couldn’t it? If so, how did you control for inclination?
In “the population of bulgeless edge-on galaxies has a similar shape to the population of face-on galaxies“, what do you mean by “similar shape”? Shape of what?
In the bottom diagram (actually a pair of figures), what is the black dotted line? In all the diagrams, what do the colors of regions of the plots mean? There’s a (horizontal) legend, which has numbers on it, but no units; there’s also a vertical legend which seems to have nothing to do with the plots at all.
Are the ‘r magnitude’s corrected for galactic extinction? k-corrected? What cosmological model did you use, with what values?
I guess my comment earned this week’s lead balloon award! 😦
Zach, if you say this, but have no intention of doing what you say, wouldn’t it be better simply to remain silent?
OTOH, if you’re simply busy, but will certainly be responding when you have a chance, might I suggest a quick “Glad to read your comment Jean! I’m busy right now, but I’ll definitely try to answer your questions when I have time.“?
Dear Jean,
I’ll answer what questions I can. Zach is a full-time student at another institution and doesn’t get updates when comments are posted, so I doubt that he has seen your questions yet.
First: yes, this can specifically extinction-corrected (u’ – r’). Color-morphology relations are seen in many magnitude pairings, though. It’s more of a general statement.
Zach’s work has controlled for edge-on vs. face-on disks using the GZ2 data, which we’ve calibrated against the axis ratios in SDSS. His work on spirals is for face-on disks only.
In the CMD space, the distributions of both colors and magnitudes for bulgeless, edge-on galaxies and that of bulgeless, face-on galaxies are similar. A lot of our work has been finding better ways of quantifying how similar the distributions are, building on things like just comparing the mean and variance.
The black dotted lines are simple fits along the most populated bins in each magnitude bin; we also worked a lot on doing more formal fits to more formal bivariate distributions. We’re not claiming yet that these lines show anything really of use, but they help draw the eye to the outline of the galaxies. Colors show the number of galaxies in each bin, weighted by the probability of the morphological feature. The hor/ver legends are the same.
The magnitudes are extinction and k-corrected, with WMAP7 values. Choice of cosmology doesn’t make a very big difference in the SDSS main galaxy sample, though.
This is work in progress, so please take it with a grain of salt. Zach did an excellent job working on the Zooniverse, but none of this has been published yet. We’re excited that more analysis will hopefully lead to papers and formal results, but much of this may change in the future.
Thank you very much Kyle!
Zach seems keen to have a discussion; is there a way I can notify him (send him an email perhaps)?
Might it be better to have the discussion in GZ Talk? or the GZ forum? The GZ blog’s comments are extremely limited in their capabilities (unless you know what HTML tags work, and what don’t).
Do you mind if I ask, what tool/app did you use to produce the colorized plots? The color scheme reminds me of elevations in atlases (all those ‘white peaks’ in WA!) …
Sure – I’ll forward your comments, and let Zach get in touch with you himself. I believe he’s at a conference this week, so I don’t know if he’ll be able to respond immediately.
Spreading this on Talk (or the forum) would be excellent – it can be hard to have real discussion in blog comments, and it’s exactly what we designed Talk for. If it hasn’t been done yet, I think it’d be ideal to cross-post the main content of the post, plus your questions – Zach, I, and the rest of the GZ community can weigh in more there.
I did just as you recommended, Kyle; the GZ Talk thread is Discussion of: Galaxy Zoo and undergraduate research: spiral arms, colors, and brightnesses. Looking forward to seeing Zach join in the discussion! 🙂
Great!